Soft magnetic alloy, dust core, and magnetic device

ABSTRACT

A soft magnetic alloy includes a main body and a surface layer. The main body has a soft magnetic alloy composition including Fe and Co. The surface layer is located on a surface side of the main body. A ratio of Co concentration to a sum of Co concentration and Fe concentration in the surface layer is Co/(Fe+Co). A distribution of Co/(Fe+Co) in a thickness direction of the surface layer includes a local minimum point and one or more local maximum points.

TECHNICAL FIELD

The present disclosure relates to a soft magnetic alloy, a dust core, and a magnetic device.

BACKGROUND

Magnetic devices such as inductors, transformers, and choke coils are widely used in power supply circuits of various electronic devices. In recent years, reduction of energy loss in power supply circuits and improvement of power supply efficiency have been emphasized for a low-carbon society, and higher efficiency and energy saving of magnetic devices are required.

In order to satisfy the above-mentioned requirements for magnetic devices, it is essential to improve the relative permeability of the magnetic core included in the magnetic device. In order to improve the relative permeability of the magnetic core, it is necessary to increase the packing rate of the magnetic material contained in the magnetic core. Thus, in the field of magnetic devices, various attempts have been made with the aim of improving the packing rate of magnetic core. For example, Patent Document 1 discloses that the packing rate can be improved by increasing the circularity of the magnetic powder. Moreover, Patent Document 2 discloses a technique for increasing the packing rate of the magnetic material by using a mixed powder of coarse powder and fine powder.

When the packing rate of the magnetic material increases, however, the number of contact points between magnetic particles increases, and the withstand voltage of the magnetic core thus tends to decrease. That is, there is a trade-off relation between the packing rate (relative permeability) and the withstand voltage. In addition, the number of contact points for each particle differs as the packing rate increases, and the difference in the number of contact points increases the variation in withstand voltage, and the m value, which indicates the degree of variation, tends to decrease. Therefore, there is a demand for the development of a technique for obtaining a high withstand voltage and a high m value even when the packing rate of the magnetic material is increased.

Patent Document 1: JP2018073947 (A)

Patent Document 2: JP2016012630 (A)

SUMMARY

The present disclosure has been achieved under such circumstances. It is an object of the present disclosure to provide a soft magnetic alloy, a dust core, and a magnetic device capable of achieving a high withstand voltage and a high m value.

To achieve the above object, a soft magnetic alloy according to the present disclosure comprises:

a main body having a soft magnetic alloy composition including Fe and Co; and

a surface layer located on a surface side of the main body,

wherein

a ratio of Co concentration to a sum of Co concentration and Fe concentration in the surface layer is Co/(Fe+Co), and

a distribution of Co/(Fe+Co) in a thickness direction of the surface layer includes a local minimum point and one or more local maximum points.

When the soft magnetic alloy having the above-mentioned characteristics is used, the withstand voltage and the m value can be improved more than before with a high relative permeability.

Preferably, the local minimum point and the one or more local maximum points in the distribution of Co/(Fe+Co) satisfies a predetermined positional relation.

For example, preferably, the local minimum point is located closer to a surface side of the surface layer than a first local maximum point as a local maximum point closest to the main body among the one or more local maximum points, and a second local maximum point as a second closest local maximum point to the main body after the first local maximum point is located closer to the surface side of the surface layer than the local minimum point.

Instead, the local minimum point may be located closest to an alloy center side among the local minimum point and the one or more local maximum points.

Preferably, the surface layer comprises an oxide phase.

Preferably, the surface layer comprises an oxide phase including at least one predetermined element M selected from Si, Cr, and Al, and the local minimum point exists in the oxide phase.

As mentioned above, when the surface layer comprises an oxide phase of the predetermined element M, the oxide phase includes a local maximum point L^(M) _(max) of concentration of the predetermined element M.

Then, preferably, one of the local maximum points for Co/(Fe+Co) exists closer to a surface side of the surface layer than the local maximum point L^(M) _(max).

The use of the soft magnetic alloy according to the present disclosure is not limited and can be applied to various magnetic devices. For example, the soft magnetic alloy according to the present disclosure can be favorably used as a dust core material in magnetic devices, such as inductors, transformers, and choke coils.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is an enlarged schematic cross-sectional view illustrating the vicinity of the surface of a soft magnetic alloy according to an embodiment of the present disclosure;

FIG. 2A is a graph representing an example of line analysis data for the soft magnetic alloy according to the embodiment;

FIG. 2B is a graph representing an example of line analysis data for the soft magnetic alloy according to the embodiment;

FIG. 3 is a graph representing a modified example of line analysis data;

FIG. 4 is a graph representing a modified example of line analysis data;

FIG. 5A is a graph representing an example of line analysis data for a conventional soft magnetic alloy;

FIG. 5B is a graph representing an example of line analysis data for a conventional soft magnetic alloy;

FIG. 6 is a schematic cross-sectional view illustrating an example of a dust core including the soft magnetic alloy shown in FIG. 1 ; and

FIG. 7 is a cross-sectional view illustrating an example of a magnetic device including a dust core.

DETAILED DESCRIPTION

Hereinafter, the present disclosure is explained in detail based on an embodiment shown in the figures.

A soft magnetic alloy 1 of the present embodiment can have a ribbon shape, a powder shape, a block shape, or the like and particularly preferably has a powder shape. The dimensions of the soft magnetic alloy 1 are not limited. For example, when the soft magnetic alloy 1 has a ribbon shape, the thickness of the soft magnetic alloy ribbon can be 15 μm to 100 μm. When the soft magnetic alloy 1 has a powder shape, the average particle size of the soft magnetic alloy powder can be 0.5 μm to 150 μm, preferably 0.5 μm to 25 μm.

The above-mentioned average particle size can be measured by various particle size analyzing methods, such as a laser diffraction method, but is preferably measured by a particle image analyzer Morphologi G3 (made by Malvern Panalytical Ltd). In Morphologi G3, the soft magnetic alloy powder is dispersed using air, and a projected area of the individual particles constituting the powder is measured so as to obtain a particle size distribution by circle equivalent diameters from the projected areas. Then, in the obtained particle size distribution, the average particle size is a particle size where a volume-based or number-based cumulative relative frequency is 50%. When the soft magnetic alloy powder is included in the magnetic core, the average particle size is obtained by measuring the circle equivalent diameters of each particle included in the cross section of the magnetic core by cross-sectional observation using an electron microscope (SEM, STEM, or the like).

FIG. 1 is a main-part cross-sectional view in which the vicinity of the surface of the soft magnetic alloy 1 is enlarged. As shown in FIG. 1 , the soft magnetic alloy 1 includes a main body 2 and a surface layer 10 located on the surface side of the main body 2. In the present embodiment, “surface side” means the side closer to the outside of the soft magnetic alloy 1 in the direction from the center of the alloy toward the surface of the alloy.

(Main Body 2)

The main body 2 is a base portion that occupies at least 90 vol % or more of the volume of the soft magnetic alloy 1. Thus, the average composition of the soft magnetic alloy 1 can be regarded as the composition of the main body 2, and the crystal structure of the soft magnetic alloy 1 can be regarded as the crystal structure of the main body 2. The volume ratio of the main body 2 can be substituted for the area ratio, and at least 90% or more of the cross-sectional area of the soft magnetic alloy 1 is the main body 2.

The main body 2 has a soft magnetic alloy composition including Fe and Co, and a specific alloy composition is not limited. For example, the main body 2 can have a crystal type soft magnetic alloy composition of a Fe—Co based alloy, a Fe—Co—V based alloy, a Fe—Co—Si based alloy, a Fe—Co—Si—Al based alloy, or the like. Instead, from the point of lowering a coercivity, the main body 2 is preferably constituted by an amorphous alloy composition or a nanocrystal alloy composition.

As an amorphous or nanocrystal soft magnetic alloy, a Fe—Co—P—C based alloy, a Fe—Co—B based alloy, a Fe—Co—B—Si based alloy, or the like may be mentioned. More specifically, the main body 2 is preferably constituted by an alloy composition satisfying a compositional formula of (Fe_((1−(α+β)))Co_(α)Niβ)_((1−(a+b)))X1_(a)X2_(b). When the main body 2 is constituted by the alloy composition satisfying the above-compositional formula, a crystal structure made of amorphous, heteroamorphous, or nanocrystals tends to be obtained easily.

In the above-mentioned compositional formula, X1 is one or more elements selected from B, P, C, Si, and Al, and X2 is one or more elements selected from Ti, Zr, Hf, Nb, Ta, Mo, W, Cr, Ga, Ag, Zn, S, Ca, Mg, V, Sn, As, Sb, Bi, N, O, Au, Cu, rare earth elements, and platinum group elements. The rare earth elements include Sc, Y, and lanthanoids. The platinum group elements include Ru, Rh, Pd, Os, Ir, and Pt. Also, α, β, a, and b represent atomic ratios, and these atomic ratios preferably satisfy the following relations.

The Co content (α) with respect to Fe is within a range of 0.005≤α≤0.700, may be within a range of 0.010≤α≤0.600, may be within a range of 0.030≤α≤0.600, or may be within a range of 0.050≤α≤0.600. When the Co content (α) is within the above-mentioned range, the saturation magnetic flux density (Bs) and the corrosion resistance of the soft magnetic alloy 1 are improved. From the point of improving Bs, 0.050≤α≤0.500 is preferably satisfied. As the Co content (α) increases, the corrosion resistance tends to improve. When the Co content (α) is too large, however, Bs tends to decrease easily.

Also, the Ni content (β) with respect to Fe may be within a range of 0≤β≤0.200. That is, the soft magnetic alloy may not include Ni, and the Ni content (β) may be within a range of 0.005≤β≤0.200. From the point of improving Bs, the Ni content (β) may be within a range of 0≤β≤0.050, may be within a of 0.001≤β≤0.050, or may be within a range of 0.005≤β≤0.010. As the Ni content (β) increases, the corrosion resistance tends to improve. When the Ni content (β) is too large, however, Bs decreases.

When a sum of atomic ratios of elements constituting the soft magnetic alloy is 1, an atomic ratio (1−(a+b)) of a total amount of Fe, Co, and Ni is preferably within a range of 0.720≤(1−(a+b))≤0.950 and is more preferably within a range of 0.780≤(1−(a+b))≤0.890. When the above-mentioned relation is satisfied, Bs tends to improve easily. When 0.720≤(1−(a+b))≤0.890 is satisfied, amorphous is easily obtained, and the coercivity tends to decrease.

X1 may be included as impurities or may be added intentionally. The X1 content (a) may be within a range of 0≤a≤0.200. From the point of improving Bs, 0≤a≤0.150 is preferably satisfied.

X2 may be included as impurities or may be added intentionally. The X2 content (b) may be within a range of 0≤b≤0.200. From the point of improving Bs, 0≤b≤0.150 is preferably satisfied, and 0≤b≤0.100 is more preferably satisfied.

The composition of the above-mentioned main body 2 (i.e., the composition of the soft magnetic alloy 1) can be analyzed, for example, by inductively coupled plasma (ICP). Here, when it is difficult to determine an oxygen amount by ICP, an impulse heat melting extraction method can also be used. If it is difficult to determine a carbon amount and a sulfur amount by ICP, an infrared absorption method can also be used.

Except for ICP, a compositional analysis may be carried out by energy dispersive X-ray spectroscopy (EDX) or electron probe microanalyzer (EPMA) attached to an electron microscope. For example, regarding the soft magnetic alloy 1 included in a dust core containing a resin component, a compositional analysis by ICP may be difficult in some cases. In this case, the compositional analysis may be carried out using EDX or EPMA. If a detailed compositional analysis is difficult by any of the above-mentioned methods, the compositional analysis may be performed using three dimensional atom probe (3DAP). When 3DAP is used, the composition of the main body 2 can be measured without the influence of the resin component, a surface oxidation, and the like in the area of analysis. This is because 3DAP can measure an average composition by determining a small area (e.g., an area of φ 20 nm×100 nm) in the soft magnetic alloy 1.

Note that, when a cross section near the surface of the soft magnetic alloy 1 is analyzed by a line analysis using EDX or electron energy loss spectroscopy (EELS), the main body 2 can be recognized as an area having stable concentrations of Fe and Co (see FIG. 2A). For example, the average composition obtained by a mapping analysis of the main body 2 can be considered as the composition of the soft magnetic alloy 1. In this case, the mapping analysis is performed using EDX or EELS. At this time, an area to be measured is an area that is 100 nm or more away in a depth direction from the surface of the soft magnetic alloy 1 (an area corresponding to the main body 2), and an area of measurement is about 256 nm×256 nm.

The crystal structure of the main body 2 (i.e., the crystal structure of the soft magnetic alloy 1) can be a crystalline structure, a nanocrystal structure, or an amorphous structure and is preferably a nanocrystal structure or an amorphous structure from the point of lowering the coercivity. For example, an amorphous degree X of the main body 2 is preferably 85% or more. The crystal structure having an amorphous degree X of 85% or more is a structure that is mostly comprised of amorphous or heteroamorphous. The structure comprised of heteroamorphous is a structure in which crystals slightly exist inside amorphous. That is, in the present embodiment, an “amorphous” is a crystal structure having an amorphous degree X of 85% or more and means that crystals may be included in a range where this amorphous degree X is satisfied.

Note that, when the structure is heteroamorphous, the average crystal grain size of crystals existing in the amorphous structure is preferably within the range of 0.1 nm or more and 10 nm or less. In the present embodiment, “nanocrystal” means a crystal structure having an amorphous degree X of less than 85% and an average crystal grain size of 100 nm or less (preferably, 3 nm to 50 nm), and “crystalline” means a crystal structure having an amorphous degree X of less than 85% and an average crystal grain size of larger than 100 nm.

The amorphous degree X can be measured by X-ray crystallography using XRD. Specifically, 2θ/θ measurement is performed using XRD to the soft magnetic alloy 1, and an X-ray diffraction chart is obtained. At this time, a measurement range of diffraction angle 2θ may be set so that an amorphous-derived halo pattern can be confirmed. For example, it is preferable to set 2θ in a range including 30° to 60°.

Next, the X-ray diffraction chart is profile-fitted using a Lorentz function represented by the following equation (2). In this profile fitting, a difference between the integrated intensities actually measured by XRD and the integrated intensities calculated using the Lorentz function is preferably determined within 1%. As a result of this profile fitting, a crystal scattering integrated intensity Ic and an amorphous scattering integrated intensity Ia are obtained. Then, the amorphous degree X is obtained by placing the crystal scattering integrated intensity Ic and the amorphous scattering integrated intensity Ia in the following equation (1).

X=100−(Ic/(Ic+Ia)×100)  Equation (1)

Ic: crystal scattering integrated intensity

[Formula1] $\begin{matrix} {{f(x)} = {\frac{h}{1 + \frac{\left( {x - u} \right)^{2}}{w^{2}}} + b}} & \left( {{Equation}2} \right) \end{matrix}$

h: peak height

u: peak position

w: half bandwidth

b: background height

Note that, a method of measuring the amorphous degree X is not limited to the above-mentioned method using XRD, and the amorphous degree X may be measured by electron backscatter diffraction (EBSD) or electron diffraction.

(Surface Layer 10)

The surface layer 10 is an area where the content rate of constituent elements of the soft magnetic alloy, such as Fe and Co, is different from that in the main body 2. The surface layer 10 covers at least a part of periphery of the main body 2. The coverage of the surface layer 10 with respect to the main body 2 in the cross section of the soft magnetic alloy 1 is not limited and can be, for example, 50% or more, preferably 80% or more.

The surface layer 10 can be analyzed by observing a cross section near the surface of the soft magnetic alloy 1 with a scanning transmission electron microscope (STEM) or a transmission electron microscope (TEM) and performing a line analysis using EDX or EELS at that time. In the line analysis, as shown in FIG. 1 , a measurement line ML is drawn along a direction substantially perpendicular to the alloy surface, and a component analysis is performed at predetermined intervals on the measurement line to obtain a concentration distribution of constituent elements near the surface. At this time, the measurement intervals for component analysis are preferably 1 nm, and the raw data measured at 1 nm intervals is preferably averaged to remove noise. More specifically, in the averaging process, an interval average value is preferably obtained at each measurement point. For example, the interval average value at a certain measurement point may be calculated by averaging the measurement values of five points, including the certain measurement point, two forward points adjacent to the certain measurement point, and two rear points adjacent to the certain measurement point. Then, the interval average values at each of the measurement points are plotted to obtain a concentration distribution graph.

For example, the graphs shown in FIG. 2A and FIG. 2B are an example of line analysis data near the surface of the soft magnetic alloy 1. For convenience of explanation, two graphs (FIG. 2A and FIG. 2B) are shown, but both of FIG. 2A and FIG. 2B show the same measurement example. The horizontal axis of each graph is the distance from a specific point (interface 21). The direction from the specific point to the alloy surface side (outer side) is the positive direction, and the direction from the specific point to the alloy inner side is the negative direction. The first vertical axis of each graph is the content rate of constituent elements (Fe and Co), and the second vertical axis of each graph is the ratio of Co concentration to the sum of Co concentration and Fe concentration: Co/(Fe+Co).

As shown in FIG. 2A, in the main body 2, the concentrations of constituent elements of Fe, Co, and the like are stable within the range of average concentration ±1 at %. On the surface side of the main body 2, there is a variation region in which the concentrations of the constituent elements are different from those of the main body 2, and this variation region is the surface layer 10. In the present embodiment, a change point CP_(Fe) in the concentration distribution of Fe, a change point CP_(Co) in the concentration distribution of Co, and a change point CP_(R) in the concentration distribution of Co/(Fe+Co) are determined, and the change point located on the innermost side of the alloy (alloy center side) among the change points is determined as an “interface 21” between the main body 2 and the surface layer 10.

Specifically, a method for determining the change points and the interface 21 is described. For example, a horizontal line AL_(Fe) corresponding to the average concentration of Fe in the main body 2 is drawn in the concentration distribution of Fe. Then, an approximation straight line TL_(Fe) is drawn in a region where the concentration of Fe monotonically changes (monotonically decreases in FIG. 2A) from the main body 2 toward the alloy surface side. The intersection between the horizontal line AL_(Fe) and the approximate straight line TL_(Fe) is a change point CP_(Fe) in the concentration distribution of Fe. A change point CP_(Co) in the concentration distribution of Co and a change point CP_(R) in the concentration distribution of Co/(Fe+Co) are determined in the same manner as described above.

The “interface 21” between the main body 2 and the surface layer 10 is determined based on the change points CP_(Fe), CP_(Co), and CP_(R) determined by the above-mentioned method. In FIG. 2A, the change point CP_(R) of Co/(Fe+Co) is located on the innermost side among the change points CP_(Fe), CP_(Co), and CP_(R). Thus, in the graph of FIG. 2A, the position where the change point CP_(R) of Co/(Fe+Co) exists is defined as the interface 21, and the interface 21 is determined as the zero point on the horizontal axis of the graph.

In the soft magnetic alloy 1 of the present embodiment, the distribution of Co/(Fe+Co) in the thickness direction of the surface layer 10 has a local minimum point L_(min) and one or more maximum points maximum points L_(max). In the graph of FIG. 2B, the local minimum point L_(min) is indicated by a black-painted circle, and the local maximum points L_(max) are indicated by a white blank circle.

Here, the local minimum point in the present embodiment is a point at which the distribution of Co/(Fe+Co) switches from a decreasing tendency to an increasing tendency in the positive direction from the interface 21 toward the surface side. That is, the local minimum point L_(min) is an extreme value in a local region where Co/(Fe+Co) changes like a valley. The local minimum point L_(min) is different from the global minimum value in the entire surface layer 10. The local maximum point in the present embodiment is a point at which the distribution of Co/(Fe+Co) switches from an increasing tendency to a decreasing tendency in the positive direction from the interface 21 toward the surface side. That is, the local maximum point L_(max) is an extreme value in a local region where Co/(Fe+Co) changes convexly. A plurality of local maximum points L_(max) may exist, and the local maximum points L_(max) and the global maximum value in the entire surface layer 10 do not necessarily correspond with each other.

Since the distribution of Co/(Fe+Co) in the surface layer 10 has a local minimum point L_(min) and one or more local maximum points L_(max), the magnetic core including the soft magnetic alloy 1 of the present embodiment can improve the withstand voltage with a high relative permeability. In addition, variations in withstand voltage can be reduced (i.e., m value can be increased), and magnetic devices can be produced stably.

The number of local maximum points L_(max) and the arrangement of each local extreme value (L_(min), L_(max)) are not limited. In particular, however, the local minimum point L_(min) and the local maximum points L_(max) exist in the manner shown in FIG. 2B. Specifically, in the graph of FIG. 2B, the distribution of Co/(Fe+Co) has two local maximum points L_(max). Among the two local maximum points L_(max), the local maximum point closest to the main body 2 (i.e., the local maximum point located on the innermost side of the alloy) is defined as a first local maximum point L1 _(max), and the local maximum point second closest to the main body 2 after the first local maximum point L1 _(max) (i.e., the maximum point closest to the alloy surface) is defined as a second local maximum point L2 _(max). In this case, preferably, the local minimum point L_(min) is located between the first local maximum point L1 _(max) and the second local maximum point L2 _(max). In other words, preferably, the local minimum point L_(min) is located closer to the alloy surface than the first local maximum point L1 _(max), and the second local maximum point L2 _(max) is located closer to the alloy surface than the local minimum point L_(min).

In the surface layer 10 having the distribution of Co/(Fe+Co) of FIG. 2B, D1 is a distance from the interface 21 to the first local maximum point L1 _(max), D2 is a distance from the interface 21 to the local minimum point L_(min), and D3 is a distance from the interface 21 to the second local maximum point L2 _(max). The distance (D1 to D3) to each local extreme value is not limited. For example, D1 is preferably 10 nm or less, D2 is preferably 20 nm or less, and D3 is preferably 30 nm or less. Each interval (D2-D1, D3-D2) between the local extreme values is not limited and is preferably, for example, 1 nm or more and 10 nm or less.

The distribution of Co/(Fe+Co) in the surface layer 10 of the soft magnetic alloy 1 is not limited to the mode shown in FIG. 2B. For example, the surface layer 10 may have the distribution of Co/(Fe+Co) as shown in FIG. 3 . In the graph of FIG. 3 , the local maximum point L_(max) does not exist between the interface 21 and the local minimum point L_(min), and the local minimum point L_(min) among the plurality of local extreme values is located closest to the alloy center (i.e., inside the surface layer 10). In other words, the distribution of Co/(Fe+Co) shown in FIG. 3 has one local maximum point L_(max), and this one local maximum point L_(max) is located closer to the alloy surface side than the local minimum point L_(min).

A distance D2′ from the interface 21 to the local minimum point L_(min) shown in FIG. 3 is not limited, but is preferably 20 nm or less like D2 in FIG. 2B. Moreover, a distance D3′ from the interface 21 to the local maximum point L_(max) shown in FIG. 3 is not limited, but is preferably 30 nm or less like D3 in FIG. 2B. The distance between the local extreme values “D3′-D2” is not limited either, but is preferably, for example, 1 nm or more and 10 nm or less.

FIG. 2A, FIG. 2B, and FIG. 3 show the concentration distributions of Fe and Co, but the surface layer 10 may include elements constituting the average composition of the soft magnetic alloy 1, such as Si, Cr, Al, B, and P, in addition to the above-mentioned elements.

The surface layer 10 can be a metal phase, an oxide phase, a metal compound phase other than an oxide, or the like and preferably includes an oxide phase. When the surface layer 10 includes an oxide phase, a higher concentration of oxygen than in the main body 2 is detected in the surface layer 10. For example, the graphs shown in FIG. 4 are an example of line analysis data of the surface layer 10 including an oxide phase.

As shown in FIG. 4 , when the oxygen concentration in the surface layer 10 is higher than that in the main body 2, the surface layer 10 includes an oxide phase 12. When the surface layer 10 includes the oxide phase 12, the local minimum point L_(min) in the distribution of Co/(Fe+Co) exists in the oxide phase 12.

The thickness of the oxide phase 12 in the surface layer 10 is not limited. For example, in the graph of FIG. 4 , the oxide phase 12 is the range from a change point CP_(Ox) of oxygen concentration to the outer surface 10 a of the surface layer 10. The change point CP_(Ox), which is an inner starting point of the oxide phase 12, does not necessarily correspond with the interface 21 between the main body 2 and the surface layer 10 and may be located closer to the surface side than the interface 21.

The oxide phase 12 may include at least one predetermined element M selected from Si, Cr, and Al and preferably includes Si as the at least one predetermined element M. When the oxide phase 12 includes the at least one predetermined element M, the oxide phase 12 includes: a region where the concentration of the at least one predetermined element M is higher than that of the main body 2; and a local maximum point L^(M) _(max) of the at least one predetermined element M. Among the local maximum points L_(max) of the Co/(Fe+Co) distribution, preferably, the second local maximum point L2 _(max) located on the surface side is located closer to the surface side of the surface layer 10 than the local maximum point L^(M) _(max) of the at least one predetermined element M.

Since the surface layer 10 includes the oxide phase 12 as shown in FIG. 4 , the withstand voltage and the m value of the magnetic core can be further improved.

In the soft magnetic alloy 1 of the present embodiment, the thickness T of the surface layer 10 is not limited, but is, for example, preferably 1 nm or more and 30 nm or less, more preferably 5 nm or more and 20 nm or less. The thickness T of the surface layer 10 can be calculated as a distance from the interface 21 to an outer surface 10 a of the surface layer 10. In the measurement of the thickness T, the interface 21 can be determined based on the predetermined change points CP as mentioned above, and the outer surface 10 a can be determined by the following method.

For example, in the graphs of FIG. 2A and FIG. 2B, the outer surface 10 a of the surface layer 10 constitutes the outermost surface of the soft magnetic alloy 1. In this case, since the outermost surface of the soft magnetic alloy 1 can be visually recognized in a TEM image or a STEM image, the outer surface 10 a in the concentration distribution graph can be determined by comparing the TEM image or the STEM image with the concentration distribution graphs shown in FIG. 2A and FIG. 2B.

The soft magnetic alloy 1 may include an insulating layer covering the surface layer 10. The insulating layer is a coated layer formed by coating or the like after forming the surface layer 10 and has an average thickness of preferably 1 nm or more and 100 nm or less, more preferably 50 nm or less. In the TEM image or the STEM image, the insulating layer may be recognized as a region having a different contrast from that of the main body 2 and the surface layer 10. In this case, the outer surface 10 a of the surface layer 10 can be determined based on the contrast of the TEM image or the STEM image. Instead, the outer surface 10 a of the surface layer 10 may be determined based on the concentration distribution of an element E specific to the insulating layer. According to the line analysis result, the concentration of the specific element E increases in the region where the surface layer 10 is switched to the insulating layer, and the change point where the specific element E increases may thus be defined as the outer surface 10 a of the surface layer 10.

(Method of Producing Soft Magnetic Alloy 1)

Hereinafter, a method of producing the soft magnetic alloy 1 having a powder shape (soft magnetic alloy powder) is described as an example of producing methods. The soft magnetic alloy 1 according to the present embodiment can be produced by performing a predetermined surface modification treatment after producing a powder by a well-known method.

A method of producing the soft magnetic alloy powder before performing a surface modification treatment is not limited. For example, the soft magnetic alloy powder may be produced by an atomizing method, such as a water atomizing method and a gas atomizing method. Instead, the soft magnetic alloy powder may be produced by a synthesis method, such as a CVD method, using at least one or more of metal salt evaporation, reduction, and thermal decomposition. Instead, the soft magnetic alloy powder may be produced using an electrolysis method or a carbonyl method. Moreover, the soft magnetic alloy powder may be produced by pulverizing a starting alloy in the form of ribbon or thin plate. The produced powder may be appropriately classified so as to adjust the particle size of the soft magnetic alloy powder.

The constituent particles of the soft magnetic alloy powder are not subjected to a predetermined surface modification treatment, and the vicinity of the surface of each constituent particle has, for example, a concentration distribution as shown in FIG. 5A. As shown in FIG. 5A, in each constituent particle before the surface modification treatment, the distribution of Co/(Fe+Co) near the surface does not have a local minimum point and a local maximum point.

Conventionally, it is known to subject the soft magnetic alloy powder to a chemical conversion treatment using an aqueous phosphate solution, a silane coupling agent, or the like. Even if a coated layer is formed by a conventional chemical conversion treatment, however, the distribution of Co/(Fe+Co) near the surface of the particle body hardly changes from the mode shown in FIG. 5A. That is, it is difficult to form the surface layer 10 having the local minimum and local maximum points of Co/(Fe+Co) only by the conventional chemical conversion treatment.

Moreover, conventionally, a heat treatment is known as a surface treatment method other than the chemical conversion treatment. When an oxide layer is formed by appropriately controlling the oxygen concentration during the heat treatment, the vicinity of the surface of the particle body may have a concentration distribution as shown in FIG. 5B. The distribution of Co/(Fe+Co) in FIG. 5B has a local maximum point, but does not have a local minimum point. That is, even if a surface treatment is performed only by the heat treatment, it is difficult to form the surface layer 10 having the local minimum point and local maximum points of Co/(Fe+Co).

As a result of intensive studies, the inventors of the present disclosure have found that the surface layer 10 having a concentration distribution as shown in FIG. 2B, FIG. 3 , or FIG. 4 can be formed by subjecting a soft magnetic alloy to a surface modification treatment by a mechanochemical method in an atmosphere in which the oxygen partial pressure is controlled. Hereinafter, the mechanochemical method is described.

The mechanochemical method is a method of applying a mechanofusion apparatus to a surface modification of the soft magnetic alloy. The mechanofusion apparatus is an apparatus that is conventionally used for a coating treatment of various powders. A desired surface layer 10 can be formed uniformly even for soft magnetic alloys having different types of compositions by using a mechanofusion apparatus to form the surface phase of the soft magnetic alloy by a different method from a conventional coating treatment.

In the mechanochemical method, first, the inside of the mechanofusion apparatus is made into a desired oxidizing atmosphere. For example, the oxygen partial pressure in the apparatus can be adjusted by using a mixed gas of Ar gas and air as the atmospheric gas to be filled in the apparatus and controlling the partial pressure of Ar gas and air in the mixed gas. The oxygen partial pressure in the apparatus is, for example, preferably 100 ppm to 10000 ppm, more preferably 500 ppm to 3000 ppm, and even more preferably 500 ppm to 1000 ppm. In the mixed gas, oxygen gas may be used instead of air, and inert gas, such as nitrogen gas and helium gas, may be used instead of Ar gas.

Next, the soft magnetic alloy powder is introduced into a rotating rotor of the mechanofusion apparatus, and the rotating rotor is rotated. A press head is installed inside the rotating rotor, and when the rotating rotor is rotated, the soft magnetic alloy powder is compressed in the gap between the inner wall surface of the rotating rotor and the press head. At this time, friction occurs between the soft magnetic alloy powder and the inner wall surface of the rotating rotor, and the soft magnetic alloy powder locally heats up. Due to this frictional heat, the surface layer 10 is formed on the surface of the main body 2. The surface layer 10 has a concentration distribution as shown in FIG. 2B (=FIG. 2A), FIG. 3 , or FIG. 4 . In particular, the surface layer 10 including the oxide phase 12 is easily formed by the above-mentioned mechanochemical method.

In the mechanochemical method, it is preferable to control the oxygen partial pressure within an appropriate range and to appropriately control the rotational speed of the rotating rotor and the gap between the inner wall surface of the rotating rotor and the press head. For example, the frictional heat generated with a low rotational speed is small, and the surface layer 10 is hard to be formed. On the other hand, if the rotational speed is too large, the compressive stress applied to the powder is large, and the surface layer 10 is likely to be formed. However, if the rotational speed is too large, the main body 2 and the surface layer 10 are likely to be destroyed, and this may lead to deterioration of magnetic characteristics. In addition, if the gap between the inner wall surface of the rotating rotor and the press head is too large, the amount of frictional heat generated is small, and the surface layer 10 is hard to be formed. On the other hand, the smaller the gap between the inner wall surface of the rotating rotor and the press head is, the larger the compressive stress applied to the powder is, and the easier it is for the surface layer 10 to be formed, but the main body 2 and the surface layer 10 are more likely to be destroyed.

After performing a heat treatment as a pretreatment, the surface modification treatment by the mechanochemical method described above may be performed. In this case, the heat treatment temperature is appropriately determined according to the composition of the soft magnetic alloy powder. For example, the heat treatment temperature for amorphous based alloys can be in the range of 300° C. to 500° C., which is lower than the crystallization temperature, the heat treatment temperature for nanocrystal based alloys can be in the range of 400° C. to 700° C., and the heat treatment temperature for crystal based alloys can be in the range of 600° C. to 1000° C. Preferably, the heat treatment atmosphere in the pretreatment is an inert atmosphere using an inert gas or a reducing atmosphere using a mixed gas of an inert gas and hydrogen. The concentration distribution as shown in FIG. 3 is easily formed by performing the above-mentioned heat treatment as the pretreatment of the mechanochemical method.

The method of forming the surface layer 10 is not necessarily limited to the above-mentioned mechanochemical method. For example, it may be possible to form the surface layer 10 having predetermined characteristics by applying a CVD method.

After the surface modification by the mechanochemical method, a heat treatment may be performed in an atmosphere in which the surface structure does not change in order to remove the stress generated by the mechanochemical method.

When an insulating layer is formed on the surface layer 10, a coating treatment, such as a phosphate coating treatment, mechanical alloying, a silane coupling treatment, and hydrothermal synthesis, is performed after the surface modification treatment by the mechanochemical method. The type of the insulating layer to be formed includes phosphates, silicates, soda-lime glass, borosilicate glass, lead glass, aluminosilicate glass, borate glass, sulfate glass, or the like. For example, phosphates include magnesium phosphate, calcium phosphate, zinc phosphate, manganese phosphate, cadmium phosphate, and the like. Also, silicates include sodium silicate, and the like.

Through the above-mentioned steps, the soft magnetic alloy 1 including the surface layer 10 is obtained.

(Use of Soft Magnetic Alloy 1)

The use of the soft magnetic alloy 1 according to the present embodiment is not limited and can be applied to various magnetic devices. In particular, the soft magnetic alloy 1 can be favorably used as a dust core material in magnetic devices, such as inductors, transformers, and choke coils. Hereinafter, an example of a dust core and a magnetic device including the soft magnetic alloy 1 is described with reference to FIG. 6 and FIG. 7 .

(Dust Core 40)

A dust core 40 including the soft magnetic alloy 1 is formed to have a predetermined shape, and its outer dimensions and shape are not limited. As shown in the schematic cross-sectional view of FIG. 6 , the dust core 40 includes the magnetic powder 3 and a resin 4 as a binder and is fixed in a predetermined shape by bonding the constituent particles (1 a, 1 b) of the magnetic powder 3 via the resin 4.

The magnetic powder 3 of the dust core 40 includes main particles 1 a each including the surface layer 10, and each of the main particles 1 a is the soft magnetic alloy 1 of the present embodiment mentioned above. The magnetic powder 3 may be composed only of the main particles 1 a each including the surface layer 10, but is preferably composed by, as shown in FIG. 6 , mixing the main particles 1 a each including the surface layer 10 and the fine particles 1 b having a smaller average particle size than the main particles 1 a. In this case, the average particle size of the main particles 1 a is preferably 5 μm or more and 25 μm or less, and the average particle size of the fine particles 1 b is preferably less than 5 μm. The material of the fine particles 1 b is not limited and may be, for example, pure iron, an Fe—Ni alloy, or the like. Each of the fine particles 1 b shown in FIG. 6 includes no insulating layer, but an insulating layer may be formed on the surface of each of the fine particles 1 b.

The ratio of the main particles 1 a (soft magnetic alloy 1) and the fine particles 1 b in the dust core 40 is not limited. For example, the mass ratio indicated by “main particles 1 a:fine particles 1 b” can be in the range of 10:90 to 90:10, preferably in the range of 60:40 to 90:10.

The material of the resin 4 is not limited and may be, for example, a thermosetting resin, such as epoxy resin. The content rate of the resin 4 in the dust core 40 is not limited and is preferably, for example, 1.0 mass % to 2.5 mass %.

The packing rate of the magnetic powder 3 in the dust core 40 can be controlled by the manufacturing conditions, such as compacting pressure, the content rate of the resin 4, or the like and can be, for example, 70 vol % to 90 vol %. From the point of increasing the relative permeability, the packing rate of the magnetic powder 3 is preferably 80 vol % or more.

In case of producing a dust core using a conventional soft magnetic alloy having a surface layer structure as shown in FIG. 5A or FIG. 5B, when the packing rate of the magnetic powder is high, the relative permeability is high, but the withstand voltage is low. That is, it is difficult for the conventional dust core to achieve both high relative permeability and high withstand voltage. On the other hand, in the dust core 40 of the present embodiment, since each of the main particles 1 a of the magnetic powder 3 includes the surface layer 10 as shown in FIG. 2B, FIG. 3 , or FIG. 4 , it is possible to improve withstand voltage and m value even at a high packing rate of 80 vol % or more. That is, in the dust core 40 of the present embodiment, the withstand voltage characteristic can be improved with a high relative permeability.

The method of manufacturing the dust core 40 is not limited. For example, the main particles 1 a subjected to a surface modification treatment by a mechanochemical method and the fine particles 1 b are mixed, and the resulting mixed powder and a thermosetting resin are thereafter kneaded to obtain a resin compound. Then, the resin compound is filled in a die and molded with pressure, and the thermosetting resin is thereafter cured to obtain the dust core 40 as shown in FIG. 6 .

(Magnetic Device 100)

In the magnetic device 100 shown in FIG. 7 , an element body is composed of the dust core 40 as shown in FIG. 6 . A coil 50 is embedded in the dust core 40, which is the element body, and ends 50 a and 50 b of the coil 50 are pulled out to the respective end surfaces of the dust core 40. A pair of external electrodes 60 and 80 is formed on the end surfaces of the dust core 40, and the pair of external electrodes 60 and 80 is electrically connected to the ends 50 a and 50 b of the coil 50, respectively.

Since the dust core 40 forming the element body has a good withstand voltage characteristic, the magnetic device 100 of the present embodiment is favorable for, for example, power inductors used in power supply circuits. The magnetic device including the soft magnetic alloy 1 is not limited to the mode as shown in FIG. 7 and may be formed by winding a wire around the surface of the dust core having a predetermined shape by a predetermined number of turns.

Hereinabove, an embodiment of the present disclosure is described, but the present disclosure is not limited to the above-mentioned embodiment and may be variously modified within the scope of the present disclosure.

Examples

Hereinafter, the present disclosure is described in further detail based on specific examples, but is not limited to the following examples. In the following tables, a sample number with “*” mark indicates a comparative example.

(Experiment 1)

In Experiment 1, six types of soft magnetic alloy powders (Powder A to Powder F) shown in Table 1 were produced. All of Powder A to Powder F were prepared by the following procedure.

First, pure metal raw materials of Fe, Co, and other subcomponents were prepared and weighed so as to obtain a desired composition after melting. Then, the weighed pure metal raw materials were melted by high-frequency heating in an evacuated chamber to obtain a mother alloy. Next, the produced mother alloy was heated at 1500° C. and melted again, and a powder having a predetermined composition was thereafter obtained by a high-pressure water atomizing method. After the atomization, the obtained powder was classified by a predetermined method to adjust the particle size of the powder. The average particle size (D50) of Powder A to Powder F produced by the above-mentioned method was all within the range of 15 μm to 25 μm.

TABLE 1 Powder No. Type Composition System Powder A Fe based amorphous Fe—B—Si—C Powder B FeCo based amorphous Fe—Co—B—Si—C Powder C Fe based nanocrystal Fe—Nb—B—Si—Cu Powder D FeCo based nanocrystal Fe—Co—Nb—B—Si—Cu Powder E Fe based crystalline Fe—Si Powder F FeCo based crystalline Fe—Co—Si

Next, each of Powder A to Powder F was divided into a plurality of samples, and each sample was subjected to a surface treatment under any of the conditions shown in Table 2.

In Conditions 1 to 5, the powder samples were subjected to a heat treatment while controlling the oxygen partial pressure within the range shown in Table 2. The temperature of the heat treatment was determined in an optimum range according to the composition of Powder A to Powder F.

In Conditions 6 to 10, the powder samples were subjected to a surface modification treatment by a mechanochemical method. At this time, the oxygen partial pressure in the rotating rotor was controlled within the range shown in Table 2 using AMS-Lab manufactured by Hosokawa Micron Corporation as a mechanofusion apparatus.

TABLE 2 Surface Atmosphere Oxygen Condition No. Treatment Method Gas Partial Pressure Condition 1 heat treatment Ar 20 ppm Condition 2 heat treatment Ar + O₂  100~300 ppm Condition 3 heat treatment Ar + O₂  500~1000 ppm Condition 4 heat treatment Ar + O₂ 1000~3000 ppm Condition 5 heat treatment Ar + O₂ 5000~10000 pm Condition 6 mechanochemical Ar 20 ppm Condition 7 mechanochemical Ar + O₂  100~300 ppm Condition 8 mechanochemical Ar + O₂  500~1000 ppm Condition 9 mechanochemical Ar + O₂ 1000~3000 ppm Condition 10 mechanochemical Ar + O₂ 5000~10000 pm

Next, dust cores were produced in the following procedure using the powder sample subjected to any of the surface treatment of Conditions 1 to 10. In Experiment 1, the powder sample subjected to any of Conditions 1 to 10 was a main powder, and a fine powder was mixed with the main powder to obtain a magnetic powder for the dust core. In all of the samples of Experiment 1, an Fe based soft magnetic alloy having an average particle size (D50) of 1 μm was used as the fine powder, and the mass ratio between the main powder and the fine powder was main powder:fine powder=80:20.

Then, the magnetic powder and an epoxy resin were kneaded to obtain a resin compound. In all of the samples of Experiment 1, the blending ratio between the magnetic powder and the epoxy resin was controlled so that the resin content rate in the dust core was 2.5 wt %. A toroidal green compact was obtained by filling the above-mentioned resin compound into a die and pressurizing it. At this time, the compacting pressure was within the range of 1 to 10 tons/cm² and controlled so that the packing rate of the magnetic powder was at least 80 vol % in all of the samples of Experiment 1. Then, the green compact was heated at 180° C. for 60 minutes to cure the epoxy resin in the green compact, and a dust core having a toroidal shape (outer diameter: 11 mm, inner diameter: 6.5 mm, and thickness: 2.5 mm) was obtained.

In each sample of Experiment 1, the following evaluations were performed on the prepared powder sample (main powder) and dust core.

(Analysis of Surface Layer Structure of Main Powder)

The surface structure of the soft magnetic alloy powders (Powder A to Powder F (main powders)) subjected to the predetermined surface treatment was analyzed by a line analysis using TEM-EDX. In the line analysis, a distribution of Co/(Fe+Co) near the surface of the soft magnetic alloy (main particles) was obtained, the presence or absence of a local minimum point L_(min) and a local maximum point L_(max) in the distribution was determined.

(Packing Rate of Magnetic Powder in Dust Core)

The dimensions and mass of the produced dust core were measured, and the density p of the dust core was calculated from the dimensions and mass. Moreover, the theoretical density of the dust core was calculated from the specific gravity of the magnetic powder, assuming that the dust core was composed of only the magnetic powder. Then, the packing rate of the magnetic powder in the dust core was calculated by dividing the density ρ by the theoretical density.

(Relative Permeability of Dust Core)

A polyurethane copper wire (UEW wire) was wound around the toroidal dust core. Then, an inductance of the dust core at a frequency of 100 kHz was measured using an LCR meter (4284A manufactured by Agilent Technologies), and a relative permeability (no unit) of the dust core was calculated based on the inductance.

(Withstand Voltage Characteristic of Dust Core)

In the measurement of withstand voltage characteristic, a cylindrical test core was produced in the same manner as the toroidal core, and In—Ga electrodes were formed on both end surfaces of the test core. Next, a voltage was applied to the test core using a withstand voltage tester (THK-2011ADMPT manufactured by Tama Densoku Co., Ltd.), and a voltage value when an electric current of 1 mA flowed was measured. Then, a withstand voltage of the test core was measured by dividing the measured voltage value by the length of the test core (distance between the end surfaces).

The withstand voltages were measured on 20 test cores for each sample, and an average value of the 20 test cores was taken as the withstand voltage of each sample. Then, the withstand voltage of each sample was relatively evaluated using the withstand voltage of the reference sample. Specifically, a dust core was produced using a powder not subjected to the surface treatment shown in Table 2 and used as the reference sample. Then, a sample exhibiting a withstand voltage of less than 1.3 times with respect to the withstand voltage of the reference sample was considered to be “failed (F)”, a sample exhibiting a withstand voltage of 1.3 times or more and less than 1.5 times was considered to be “good (G)”, and a sample exhibiting a withstand voltage of 1.5 times or more was considered to be “very good (VG)”.

A Weibull plot was obtained using the withstand voltage data of the 20 test cores as a population, and a m value (no unit) of each sample was calculated from the Weibull plot. The m value is an index showing the degree of variation in withstand voltage. A m value of 3.0 or more was considered to be good, and a m value of 5.5 or more was considered to be very good.

The evaluation results of each sample in Experiment 1 are shown in Tables 3-5. Table 3 shows evaluation results of samples using an amorphous based main powder (Powder A or Powder B), Table 4 shows evaluation results of samples using a nanocrystal based main powder (Powder C or Powder D), and Table 5 shows evaluation results of samples using a crystalline main powder (Powder E or Powder F). In each table, “-” in the column of surface treatment method means that the surface treatment shown in Table 2 was not performed.

TABLE 3 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value A-1* powder A — Fe 2.5 80.5 absent absent 38.1 reference 2.2 A-2* powder A condition 3 Fe 2.5 81.5 absent absent 39.4 F 2.3 A-3* powder A condition 8 Fe 2.5 80.8 absent absent 38.6 F 2.3 B-1* powder B — Fe 2.5 80.6 absent absent 38.9 reference 2.4 B-2* powder B condition 1 Fe 2.5 81.2 absent absent 39.7 F 2.5 B-3* powder B condition 2 Fe 2.5 80.7 present absent 39.1 F 2.6 B-4* powder B condition 3 Fe 2.5 80.9 present absent 39.4 F 2.7 B-5* powder B condition 4 Fe 2.5 81.4 present absent 39.8 F 2.8 B-6* powder B condition 5 Fe 2.5 81.1 present absent 39.4 F 2.9 B-7* powder B condition 6 Fe 2.5 81.3 absent absent 39.8 F 2.6 B-8 powder B condition 7 Fe 2.5 80.8 present present 39.4 G 3.1 B-9 powder B condition 8 Fe 2.5 80.7 present present 39.1 VG 5.6 B-10 powder B condition 9 Fe 2.5 81.4 present present 39.8 G 4.9 B-11 powder B condition 10 Fe 2.5 81.4 present present 39.9 G 3.2

TABLE 4 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value C-1* powder C — Fe 2.5 80.9 absent absent 51.7 reference 2.3 C-2* powder C condition 3 Fe 2.5 80.6 absent absent 50.7 F 2.4 C-3* powder C condition 8 Fe 2.5 80.7 absent absent 51.2 F 2.4 D-1* powder D — Fe 2.5 81.4 absent absent 44.6 reference 2.5 D-2* powder D condition 1 Fe 2.5 81.0 absent absent 44.2 F 2.6 D-3* powder D condition 2 Fe 2.5 81.2 present absent 45.6 F 2.7 D-4* powder D condition 3 Fe 2.5 81.3 present absent 47.4 F 2.8 D-5* powder D condition 4 Fe 2.5 81.0 present absent 44.2 F 2.9 D-6* powder D condition 5 Fe 2.5 81.3 present absent 47.4 F 3.0 D-7* powder D condition 6 Fe 2.5 80.6 absent absent 42.7 F 3.0 D-8 powder D condition 7 Fe 2.5 81.0 present present 44.7 G 3.1 D-9 powder D condition 8 Fe 2.5 81.1 present present 44.2 VG 5.7 D-10 powder D condition 9 Fe 2.5 80.6 present present 42.7 G 4.8 D-11 powder D condition 10 Fe 2.5 81.0 present present 44.7 G 3.2

TABLE 5 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value E-1* powder E — Fe 2.5 81.0 absent absent 41.2 reference 2.5 E-2* powder E condition 3 Fe 2.5 81.5 absent absent 42.7 F 2.2 E-3* powder E condition 8 Fe 2.5 81.1 absent absent 41.9 F 2.2 F-1* powder F — Fe 2.5 81.2 absent absent 36.2 reference 2.5 F-2* powder F condition 1 Fe 2.5 81.5 absent absent 36.1 F 2.6 F-3* powder F condition 2 Fe 2.5 81.2 present absent 36.3 F 2.7 F-4* powder F condition 3 Fe 2.5 80.6 present absent 35.4 F 2.8 F-5* powder F condition 4 Fe 2.5 81.2 present absent 36.2 F 2.9 F-6* powder F condition 5 Fe 2.5 80.7 present absent 35.6 F 2.9 F-7* powder F condition 6 Fe 2.5 80.6 absent absent 35.3 F 2.9 F-8 powder F condition 7 Fe 2.5 81.1 present present 36.2 G 3.1 F-9 powder F condition 8 Fe 2.5 81.0 present present 35.9 VG 6.3 F-10 powder F condition 9 Fe 2.5 80.7 present present 35.6 G 4.9 F-11 powder F condition 10 Fe 2.5 80.9 present present 35.6 G 3.3

In Samples A-2 to A-3, C-2 to C-3, and E-2 to E-3, the main powder not including Co (Powder A, Powder C, or Powder E) was subjected to the heat treatment or the mechanochemical treatment. In these samples, the withstand voltage characteristic was comparable to that of the reference sample, and the withstand voltage characteristic was not improved.

In Samples B-2 to B-6, D-2 to D-6, and F-2 to F-6, the main powder including Co (Powder B, Powder D, or Powder F) was subjected to the heat treatment. In these samples, the surface layer of the soft magnetic alloy (main particles) had a local maximum point L_(max), but did not have a local minimum point L_(min). The withstand voltage characteristic of the sample having only a local maximum point L_(max) was comparable to that of the reference sample, and the withstand voltage characteristic was not improved.

On the other hand, in Samples B-8 to B-11, D-8 to D-11, and F-8 to F-11, the main powder including Co (Powder B, Powder D, or Powder F) was subjected to the mechanochemical treatment at an oxygen partial pressure of 100 ppm to 10000 ppm. In these samples, the surface layer 10 of the soft magnetic alloy (main particles) had a local minimum point L_(min) and one or more local maximum points L_(max). As shown in Tables 3 to 5, in the above-mentioned samples each including the predetermined surface layer 10, a high relative permeability comparable to that of the reference sample was obtained, and a high withstand voltage and a high m value were obtained. This result indicates that when the surface layer 10 of the soft magnetic alloy has a local minimum point L_(min) and one or more local maximum points L_(max), the withstand voltage and the m value can be improved with a high relative permeability.

In Samples B-8 to B-11, D-8 to D-11, and F-8 to F-11, the surface layer 10 of the soft magnetic alloy (main particles) had an oxide phase including Si. In the oxide phase, there was a local maximum point L^(Si) _(max) of Si concentration, and a local maximum point L_(max) of Co/(Fe+Co) was located closer to the surface side than the local maximum point L^(Si) _(max) of Si.

(Experiment 2)

In Experiment 2, dust cores were produced using a fine powder different from that in Experiment 1 and a main powder (Powder B, Powder D, or Powder F). Specifically, in Experiment 2, the fine powder was an FeNi based soft magnetic alloy powder having an average particle size (D50) of 1 μm. In Experiment 2, the experimental conditions other than the type of fine powder were the same as those in Experiment 1, and the same evaluations as in Experiment 1 were performed. The evaluation results of Experiment 2 are shown in Tables 6-8. In addition to the results of Experiment 2, Tables 6-8 also show the evaluation results of Experiment 1 using the Fe fine powder.

TABLE 6 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value B-1* powder B — Fe 2.5 80.6 absent absent 38.9 reference 2.4 B2-1* powder B — FeNi 2.5 81.4 absent absent 39.8 F 2.4 B-2* powder B condition 1 Fe 2.5 81.2 absent absent 39.7 F 2.5 B2-2* powder B condition 1 FeNi 2.5 80.9 absent absent 39.4 F 2.5 B-3* powder B condition 2 Fe 2.5 80.7 present absent 39.1 F 2.6 B2-3* powder B condition 2 FeNi 2.5 80.9 present absent 39.4 F 2.6 B-4* powder B condition 3 Fe 2.5 80.9 present absent 39.4 F 2.7 B2-4* powder B condition 3 FeNi 2.5 80.6 present absent 38.9 F 2.7 B-5* powder B condition 4 Fe 2.5 81.4 present absent 39.8 F 2.8 B2-5* powder B condition 4 FeNi 2.5 80.5 present absent 38.9 F 2.8 B-6* powder B condition 5 Fe 2.5 81.1 present absent 39.4 F 2.9 B2-6* powder B condition 5 FeNi 2.5 81.3 present absent 39.9 F 2.9 B-7* powder B condition 6 Fe 2.5 81.3 absent absent 39.8 F 2.6 B2-7* powder B condition 6 FeNi 2.5 81.0 absent absent 39.4 F 2.6 B-8 powder B condition 7 Fe 2.5 80.8 present present 39.4 G 3.1 B2-8 powder B condition 7 FeNi 2.5 81.0 present present 39.4 G 3.1 B-9 powder B condition 8 Fe 2.5 80.7 present present 39.1 VG 5.6 B2-9 powder B condition 8 FeNi 2.5 81.1 present present 39.4 VG 5.6 B-10 powder B condition 9 Fe 2.5 81.4 present present 39.8 G 4.9 B2-10 powder B condition 9 FeNi 2.5 81.0 present present 39.4 G 4.9 B-11 powder B condition 10 Fe 2.5 81.4 present present 39.9 G 3.2 B2-11 powder B condition 10 FeNi 2.5 80.7 present present 39.1 G 3.2

TABLE 7 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value D-1* powder D — Fe 2.5 81.4 absent absent 44.6 reference 2.5 D2-1* powder D — FeNi 2.5 81.0 absent absent 44.2 F 2.5 D-2* powder D condition 1 Fe 2.5 81.0 absent absent 44.2 F 2.6 D2-2* powder D condition 1 FeNi 2.5 81.1 absent absent 44.2 F 2.6 D-3* powder D condition 2 Fe 2.5 81.2 present absent 45.6 F 2.7 D2-3* powder D condition 2 FeNi 2.5 81.1 present absent 44.2 F 2.7 D-4* powder D condition 3 Fe 2.5 81.3 present absent 47.4 F 2.8 D244* powder D condition 3 FeNi 2.5 80.8 present absent 42.6 F 2.8 D-5* powder D condition 4 Fe 2.5 81.0 present absent 44.2 F 2.9 D2-5* powder D condition 4 FeNi 2.5 80.6 present absent 42.7 F 2.9 D-6* powder D condition 5 Fe 2.5 81.3 present absent 47.4 F 3.0 D2-6* powder D condition 5 FeNi 2.5 81.3 present absent 45.6 F 3.0 D-7* powder D condition 6 Fe 2.5 80.6 absent absent 42.7 F 3.0 D2-7* powder D condition 6 FeNi 2.5 81.0 absent absent 44.7 F 3.0 D-8 powder D condition 7 Fe 2.5 81.0 present present 44.7 G 3.1 D2-8 powder D condition 7 FeNi 2.5 81.4 present present 47.4 G 3.1 D-9 powder D condition 8 Fe 2.5 81.1 present present 44.2 VG 5.7 D2-9 powder D condition 8 FeNi 2.5 81.0 present present 44.2 VG 5.7 D-10 powder D condition 9 Fe 2.5 80.6 present present 42.7 G 4.8 D2-10 powder D condition 9 FeNi 2.5 81.4 present present 44.6 G 4.8 D-11 powder D condition 10 Fe 2.5 81.0 present present 44.7 G 3.2 D2-11 powder D condition 10 FeNi 2.5 81.0 present present 44.7 G 3.2

TABLE 8 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value F-1* powder F — Fe 2.5 81.2 absent absent 36.2 reference 2.5 F2-1* powder F — FeNi 2.5 81.2 absent absent 36.3 F 2.5 F-2* powder F condition 1 Fe 2.5 81.5 absent absent 36.1 F 2.6 F2-2* powder F condition 1 FeNi 2.5 81.1 absent absent 36.0 F 2.6 F-3* powder F condition 2 Fe 2.5 81.2 present absent 36.3 F 2.7 F2-3* powder F condition 2 FeNi 2.5 80.7 present absent 35.3 F 2.7 F-4* powder F condition 3 Fe 2.5 80.6 present absent 35.4 F 2.8 F2-4* powder F condition 3 FeNi 2.5 81.1 present absent 36.2 F 2.8 F-5* powder F condition 4 Fe 2.5 81.2 present absent 36.2 F 2.9 F2-5* powder F condition 4 FeNi 2.5 80.8 present absent 35.6 F 2.9 F-6* powder F condition 5 Fe 2.5 80.7 present absent 35.6 F 2.9 F2-6* powder F condition 5 FeNi 2.5 81.4 present absent 36.0 F 2.9 F-7* powder F condition 6 Fe 2.5 80.6 absent absent 35.3 F 2.9 F2-7* powder F condition 6 FeNi 2.5 80.8 absent absent 35.6 F 2.9 F-8 powder F condition 7 Fe 2.5 81.1 present present 36.2 G 3.1 F2-8 powder F condition 7 FeNi 2.5 80.7 present present 35.6 G 3.1 F-9 powder F condition 8 Fe 2.5 81.0 present present 35.9 VG 6.3 F2-9 powder F condition 8 FeNi 2.5 81.0 present present 36.0 VG 6.3 F-10 powder F condition 9 Fe 2.5 80.7 present present 35.6 G 4.9 F2-10 powder F condition 9 FeNi 2.5 80.7 present present 35.6 G 4.9 F-11 powder F condition 10 Fe 2.5 80.9 present present 35.6 G 3.3 F2-11 powder F condition 10 FeNi 2.5 81.3 present present 36.3 G 3.3

The results of Tables 6 to 8 indicate that the relative permeability may change by changing the type of fine powder. In the sample including a local minimum point L_(min) and one or more local maximum points L_(max), even when the relative permeability was changed by the change of the fine powder type, a high withstand voltage and a high m value were obtained without decrease in withstand voltage characteristic.

(Experiment 3)

In Experiment 3, the resin content rate in the dust core was changed. Specifically, an epoxy resin and a magnetic powder containing a predetermined main powder (Powder B, Powder D, or Powder F) were kneaded so that the resin content rate was 2.5 wt %, 2.0 wt %, 1.5 wt %, or 1.0 wt %. In Experiment 3, the experimental conditions other than the resin content rate were the same as those in Experiment 1, and the same evaluations as in Experiment 1 were performed. The evaluation results of Experiment 3 are shown in Tables 9-11.

TABLE 9 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value B-1* powder B — Fe 2.5 80.6 absent absent 38.9 reference 2.4 B3-1* powder B — Fe 2.0 81.9 absent absent 40.6 F 2.2 B3-2* powder B — Fe 1.5 83.2 absent absent 42.2 F 2.0 B3-3* powder B — Fe 1.0 84.5 absent absent 44.0 F 1.8 B-9 powder B condition 8 Fe 2.5 80.7 present present 39.1 VG 5.6 B3-4 powder B condition 8 Fe 2.0 82.5 present present 41.5 VG 6.0 B3-5 powder B condition 8 Fe 1.5 83.7 present present 42.8 VG 5.8 B3-6 powder B condition 8 Fe 1.0 85.1 present present 44.4 VG 5.7

TABLE 10 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value D-1* powder D — Fe 2.5 81.4 absent absent 44.6 reference 2.5 D3-1* powder D — Fe 2.0 82.3 absent absent 46.9 F 2.2 D3-2* powder D — Fe 1.5 83.6 absent absent 50.1 F 2.0 D3-3* powder D — Fe 1.0 85.0 absent absent 51.1 F 1.9 D-9 powder D condition 8 Fe 2.5 81.1 present present 44.2 VG 5.7 D3-4 powder D condition 8 Fe 2.0 82.1 present present 46.0 VG 6.1 D3-5 powder D condition 8 Fe 1.5 83.2 present present 46.9 VG 6.0 D3-6 powder D condition 8 Fe 1.0 84.7 present present 50.8 VG 5.9

TABLE 11 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fine Resin Packing Local Local Characteristics Sample Surface Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value F-1* powder F — Fe 2.5 81.2 absent absent 36.2 reference 2.5 F3-1* powder F — Fe 2.0 82.3 absent absent 36.9 F 2.2 F3-2* powder F — Fe 1.5 83.4 absent absent 37.9 F 2.0 F3-3* powder F — Fe 1.0 84.4 absent absent 38.6 F 1.9 F-9 powder F condition 8 Fe 2.5 81.0 present present 35.9 VG 6.3 F3-4 powder F condition 8 Fe 2.0 82.5 present present 37.3 VG 6.1 F3-5 powder F condition 8 Fe 1.5 83.9 present present 38.3 VG 6.0 F3-6 powder F condition 8 Fe 1.0 85.2 present present 39.4 VG 5.9

As shown in Tables 9-11, in the samples not subjected to a predetermined mechanochemical treatment (the sample not including the surface layer 10), the relative permeability was improved by reducing the resin content rate, but the withstand voltage and the m value were reduced. On the other hand, in the samples subjected to a predetermined mechanochemical treatment (the sample including the surface layer 10), a high withstand voltage and a high m value were obtained even though the resin content rate was reduced. That is, the sample including a local minimum point L_(min) and one or more local maximum points L_(max) can achieve both a high relative permeability and a high withstand voltage characteristic even when the resin content rate is reduced.

(Experiment 4)

In Experiment 4, an insulating layer composed of a phosphate based compound was formed on each particle surface of the main powder (Powder B, Powder D, or Powder F) by phosphate treatment. Specifically, samples each including only the insulating layer without performing a mechanochemical treatment (B4-1, D4-1, and F4-1) and samples each including the insulating layer after performing a mechanochemical treatment (B4-2, D4-2, and F4-2) were prepared. In all of the samples of Experiment 4, the average thickness of the insulating layer was within the range of 1 nm to 50 nm, and the resin content rate was 1.0 wt %. In Experiment 4, the experimental conditions other than the above were the same as those in Experiment 1, and the same evaluations as in Experiment 1 were performed. The evaluation results of Experiment 4 are shown in Table 12.

TABLE 12 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Main Powder Fire Resin Packing Local Local Characteristics Sample Surface Insulating Powder Content Rate Rate Maximum Minimum Relative Withstand No. Type Treatment Layer Type wt % vol % Point: L_(max) Point: L_(min) Permeability Voltage m Value B3-3* powder B — absent Fe 1.0 84.5 absent absent 44.0 reference 1.8 B4-1* powder B — present Fe 1.0 84.3 absent absent 43.5 G 2.1 B3-6 powder B condition 8 absent Fe 1.0 85.1 present present 44.4 VG 5.7 B4-2 powder B condition 8 present Fe 1.0 84.9 present present 44.3 VG 7.2 D3-3* powder D — absent Fe 1.0 85.0 absent absent 51.1 reference 1.9 D4-1* powder D — present Fe 1.0 84.8 absent absent 51.6 G 2.1 D3-6 powder D condition 8 absent Fe 1.0 84.7 present present 50.8 VG 5.9 D4-2 powder D condition 8 present Fe 1.0 84.5 present present 49.3 VG 7.8 F3-3* powder F — absent Fe 1.0 84.4 absent absent 38.6 reference 1.9 F4-1* powder F — present Fe 1.0 84.2 absent absent 38.5 G 2.1 F3-6 powder F condition 8 absent Fe 1.0 85.2 present present 39.4 VG 5.9 F4-2 powder F condition 8 present Fe 1.0 85.0 present present 39.2 VG 7.9

The results in Table 12 indicate that the withstand voltage characteristic was further improved by further forming an insulating layer on the outer surface of the surface layer 10.

(Experiment 5)

In Experiment 5, a heat treatment was performed as a pretreatment, and a mechanochemical treatment was thereafter performed. Specifically, the conditions for the heat treatment were heat treatment temperature: 300° C., atmosphere: an inert atmosphere of Ar gas, and Powder B was subjected to the heat treatment under these conditions. After the heat treatment, a mechanochemical treatment was performed under Condition 8 in Table 2. In Experiment 5, the experimental conditions other than the above were the same as those in Experiment 1, and the same evaluations as in Experiment 1 were performed. The evaluation results of Experiment 5 are shown in Table 13. Together with the results of Experiment 5, Table 13 also shows evaluation results of Experiment 1 (Samples B-1, B-4, and B-9) without the pretreatment.

TABLE 13 Magnetic Powder Dust Core Co/(Fe + Co) Distribution Fine Resin Packing Local Local Characteristics Sample Main Powder Powder Content Rate Rate Maximum Number of Local Minimum Relative Withstand m No. Type Surface Treatment Type wt % vol % Point: L_(max) Maximum Points Point: L_(min) Permeability Voltage Value B-1* powder B — Fe 2.5 80.6 absent 0 absent 38.9 reference 2.4 B-4* powder B conditions Fe 2.5 80.9 present 1 absent 39.4 F 2.7 B-9 powder B conditions Fe 2.5 80.7 present 2 present 39.1 VG 5.6 B5-1 powder B heat treatment + Fe 2.5 81.3 present 1 present 39.8 VG 5.6 conditions

In Sample B-9 not subjected to the pretreatment, the surface layer 10 of the soft magnetic alloy (main particles) had two local maximum points L_(max) and a minimum point L_(min), and the local minimum point L_(min) existed between the two local maximum points L_(max) (corresponding to FIG. 2B). On the other hand, in Sample B5-1 subjected to the heat treatment as the pretreatment, the surface layer 10 of the soft magnetic alloy (main particles) had one local maximum point L_(max) and a local minimum point L_(min). In Sample B5-1, the local minimum point L_(min) was located closer to the alloy center than the local maximum point L_(max) (corresponding to FIG. 3 ). In both of Sample B-9 and Sample B5-1, a high withstand voltage and a high m value were obtained. This result indicates that the number of local maximum points L_(max) may be singular or plural, and that an excellent withstand voltage characteristic can be obtained by having a surface layer structure as shown in FIG. 2B or FIG. 3 .

DESCRIPTION OF THE REFERENCE NUMERICAL

-   1 . . . soft magnetic alloy     -   2 . . . main body     -   10 . . . surface layer         -   10 a . . . outer surface         -   12 . . . oxide phase     -   21 . . . interface -   3 . . . magnetic powder -   1 a . . . main particle -   1 b . . . fine particle -   4 . . . resin -   40 . . . dust core -   50 . . . coil     -   50 a, 50 b . . . end -   60, 80 . . . external electrode -   100 . . . magnetic device 

What is claimed is:
 1. A soft magnetic alloy comprising: a main body having a soft magnetic alloy composition including Fe and Co; and a surface layer located on a surface side of the main body, wherein a ratio of Co concentration to a sum of Co concentration and Fe concentration in the surface layer is Co/(Fe+Co), and a distribution of Co/(Fe+Co) in a thickness direction of the surface layer includes a local minimum point and one or more local maximum points.
 2. The soft magnetic alloy according to claim 1, wherein the local minimum point is located closer to a surface side of the surface layer than a first local maximum point as a local maximum point closest to the main body among the one or more local maximum points, and a second local maximum point as a second closest local maximum point to the main body after the first local maximum point is located closer to the surface side of the surface layer than the local minimum point.
 3. The soft magnetic alloy according to claim 1, wherein the local minimum point is located closest to an alloy center side among the local minimum point and the one or more local maximum points.
 4. The soft magnetic alloy according to claim 1, wherein the surface layer comprises an oxide phase.
 5. The soft magnetic alloy according to claim 1, wherein the surface layer comprises an oxide phase including at least one predetermined element selected from Si, Cr, and Al, and the local minimum point exists in the oxide phase.
 6. The soft magnetic alloy according to claim 5, wherein the oxide phase includes a local maximum point L^(M) _(max) of concentration of the at least one predetermined element, and one of the local maximum points for Co/(Fe+Co) exists closer to a surface side of the surface layer than the local maximum point L^(M) _(max).
 7. A dust core comprising the soft magnetic alloy according to claim
 1. 8. A magnetic device comprising the soft magnetic alloy according to claim
 1. 